The Prophet (ﷺ) said, "Whoever swears by a religion other than Islam (i.e. if he swears by saying that he is a non-Muslim in case he is telling a lie), then he is as he says if his oath is false and whoever commits suicide with something, will be punished with the same thing in the (Hell) fire, and cursing a believer is like murdering him, and whoever accuses a believer of disbelief, then it is as if he had killed him."
Sahih al-Bukhari Book 78, Hadith 132
← Previous Hadith Allah's Messenger (ﷺ) said, 'If anyone says to his brother, 'O misbeliever! Then surely, one of them such."

Sahih al-Bukhari Book 78, Hadith 131 Next Hadith → Mu`adh bin Jabal used to pray with the Prophet (ﷺ) and then go to lead his people in prayer. Once he led the people in prayer and recited Surat-al-Baqara. A man left (the row of the praying people) and offered (light) prayer (separately) and went away. When Mu`adh came to know about it, he said. "He (that man) is a hypocrite." Later that man heard what Mu`adh said about him, so he came to the Prophet and said, "O Allah's Messenger (ﷺ)! We are people who work with our own hands and irrigate (our farms) with our camels. Last night Mu`adh led us in the (night) prayer and he recited Sura-al-Baqara, so I offered my prayer separately, and because of that, he accused me of being a hypocrite." The Prophet called Mu`adh and said thrice, "O Mu`adh! You are putting the people to trials? Recite 'Washshamsi wad-uhaha' (91) or'Sabbih isma Rabbi ka-l-A'la' (87) or the like."

Sahih al-Bukhari Book 78, Hadith 133